1. Field of the Invention
This invention relates to a negative electrode for alkaline storage batteries based on a hydrogen absorbing alloy that repeats electrochemical absorption and desorption of hydrogen with charging and discharging of the batteries.
2. Description of the Prior Art
An alkaline storage battery is a generic name of secondary batteries that employ alkaline electrolyte solution having potassium hydroxide as the main electrolyte. Most of these battery systems use nickel oxyhydroxide (NiOOH) as the positive active material because of its superior charge-discharge cycle characteristics and long life. A nickel cadmium system which uses cadmium as the negative active material has been the main stream of sealed alkaline storage batteries with a long charge-discharge cycle life and high reliability. Recently, nickel-metal hydride (Ni--MH) based batteries have been commercialized which use a hydrogen absorbing alloy as the negative electrode which is easier to achieve higher capacity than that with a cadmium negative electrode. Although the nominal cell voltage of Ni--MH alkaline storage batteries is 1.2 volts which is the same as that of Ni--Cd systems, demand for Ni--MH batteries has been rapidly growing by replacing Ni--Cd systems because of their high capacity and environmental friendliness. Potential applications of the Ni--MH alkaline storage batteries include small size power supply for portable equipment such as portable telephone, camcorder, and lap-top computer to medium to large size power supply for electric vehicles and hybrid electric vehicles.
Negative electrode of an alkaline storage battery using hydrogen absorbing alloy repeats a cycle of electrochemically absorbing hydrogen, when charging, producing a hydride and, when discharging, desorbing hydrogen thus returning back to the initial alloy. As the hydrogen absorbing alloy for use as a negative electrode of alkaline storage batteries, LaNi.sub.5 - or MmNi.sub.5 (Mm: misch metal)-based AB.sub.5 type alloy having a CaCu.sub.5 type crystal structure in which a part of the Ni has been substituted by other element such as Co, Mn, or Al is currently in main use. In this type of alloy, Co is known to be an essential additive element as typified by an alloy composition of MmNi.sub.3.55 Co.sub.0.75 M.sub.0.4 Al.sub.0.3. Here misch metal (Mm) is a mixture of native rare earth metal elements mainly comprising lanthanum (La) and cesium (Cs) and containing praseodymium (Pr), neodymium (Nd), samarium (Sm), etc.
As the negative electrode of alkaline storage batteries, non-sintered electrodes such as electrodes fabricated by either filling a paste prepared by mixing powders of afore-mentioned AB.sub.5 type alloy, binder, and solvent into three-dimensional continuous pores having pore diameters of 100 to 300 .mu.m inside a high-porosity spongy nickel sheet, or paste type electrode fabricated by coating the paste on both sides of a grid made of nickel-plated and perforated steel sheet, are generally employed.
In the above described AB.sub.5 type hydrogen absorbing alloy based on MmNi.sub.5, Co that substituted part of the Ni has an effect of suppressing performance deterioration of the negative electrode resulting from corrosion of the alloy due to alkaline electrolyte. Cobalt is also considered to have an important function of suppressing the decrease of discharge capacity of the negative electrode and O.sub.2 gas absorbing capability due to unrecoverable oxidation caused by O.sub.2 gas evolving from the positive electrode during charge and overcharge, or due to the alloy powders becoming finer and finer as a result from repeated hydrogen absorption and desorption accompanying charge and discharge.
In alkaline storage batteries based on Ni--MH systems, further improvements in battery characteristics such as energy density, cycle life, high-rate charge-discharge, and storage performance as well as reduction in cost are problems to be solved in making this type of battery widely accepted as a general use battery.
The cost of Co to be added in the negative electrode alloy is relatively high. Also, since it is produced in limited and localized parts of the world, the supply is unstable and dependent on political situations, and the price tends to fluctuate. It is therefore desirable to minimize the quantity of Co to be added to the alloy as much as possible or preferably to do away with it. However, when the conventional added quantity of Co is reduced to 6 atomic % or less for example, it has been difficult to make a battery having a satisfactory cycle life or storage performance. It has therefore been considered necessary to add Co by 8 to 15 atomic %.
In an effort to reduce the added quantity of Co, much research has been done for a long time in the area of composition, preparation, and surface treatment of alloys. Taking alloy composition as an example, it was studied to form a fine segregation phase within microstructure by changing the ratio of A and B elements of an AB.sub.5 type alloy or by adding a new element. In making alloy powders, in place of the conventional method of mechanically pulverizing ingot obtained by casting of molten alloy in a high-frequency furnace, there is proposed an atomizing process in which molten alloy is atomized like spray by an inert gas. With these means, attempts have been made to reduce the quantity of Co addition and yet secure battery performance. As a result, some success has been obtained in securing anti-corrosiveness and resistance to oxidation of AB.sub.5 type hydrogen absorbing alloy even though quantity of Co addition is reduced by a certain degree. However, it was found difficult to obtain a superior cycle life and a long storage life in practical batteries.
Also, some proposals have been made regarding surface treatment of alloy powders. Typical examples include a method of etching the surface of alloy powders in an alkaline solution, micro-encapsulating alloy powders by plating nickel or copper on their surface, or covering the surface of alloy powders by forming fluoride layer. However, even with these surface treatments, actual batteries suffered unsatisfactory cycle life and shelf life.